This application relates generally to windings for making electrical coils, and more specifically to a Roebel winding with a conductive felt and non-conductive filler.
Windings for use in electrical coils, such as Roebel windings use fillers to fill interstices between the windings and a casing wall. Various techniques for filling voids between parts in electrically conductive devices are known. Unfortunately, many prior art techniques do not succeed in completely filling the voids and/or suppressing electrical discharge across the voids. Many void fillers act as a dielectric and allow a voltage to be impressed across the filler. Failure to fill the voids or at least suppress discharge will result in undesirable arcing between the components. Arcing leads to diminished efficiency and diminished life expectancy of the device.
An example of a conductive device where voids are present is a high voltage coil having windings that are intertwined in a braid-like fashion to form a Roebel bar. Roebel bars, or Roebelled windings, tend to have a highly discontinuous surface. Such a surface tends to have a great number of voids, or interstices, which must be properly filled in order to reduce mechanical and electrical stresses. U.S. Pat. No. 5,175,396 dated Dec. 29, 1992 to Emery, incorporated herein by reference, discloses such a Roebel bar. The U.S. Pat. No. 5,175,396 discloses a prior art void filler made from Dacron felt impregnated with epoxy. The U.S. Pat. No. 5,175,396 is directed to providing a void filler made from an insulating layer of mica paper and B stage epoxy. A semiconductive layer, preferably a paste of carbon filled epoxy, is placed between the inner insulating layer and a groundwall.
Other filler materials have been used to fill voids in electrical coils. Discussion of a resin rich felt material may be found in U.S. Pat. No. 5,633,477 dated May 27, 1997 to Smith, and incorporated by reference. Discussion of an inert filler material and a pyrolyzed glass fiber layer electrically coupled to the strands of a coil may be found in U.S. Pat. No. 5,066,881 dated Nov. 19, 1991 to Elton et al., also incorporated by reference. These fillers, and other prior art fillers and pre-pegs, are often difficult to install in high voltage coils and/or are not suitable for use in other applications, such as in the construction of an electrically shielded cabinet.
A conductive filler is disclosed in U.S. Pat. No. 6,559,384, issued on May 6, 2003 to Angell et al., and incorporated herein by reference. However, such a filler may make it difficult to properly form the coil into the desired shape, because sanding the profile of the Roebel filler to the coil shape after pre-consolidation without damaging the function of the electrical connection in the filler is difficult.
Additional prior art solutions are disclosed in U.S. Pat. No. 6,827,805 dated Dec. 7, 2004, to Angell et al. (which discloses a method of manufacture of a bar with the conductive resin filler of the '384 patent), and U.S. Pat. No. 6,677,848 B1 issued on Jan. 13, 2004, to Emery (which discloses a high-voltage winding and is described for use in a dynamoelectric machine), and U.S. Pat. No. 6,724,118 B2 issued on Apr. 20, 2004, to Emery, all incorporated herein by reference.
It can be difficult to produce a product that provides a conductive filler in the Roebel transposed area of the desired bar that can provide a conductive plane yet be sanded or machined to dimension without disrupting the fillers function. Other solid conductive fillers in the market use conductive resin and insulating mica flakes in a putty. This putty having conductive resin will short the strand to strand connections in the roebel bar as the conductive resin impregnates the insulation layer of the copper conductor roebel strands. The solution disclosed in U.S. Pat. No. 6,559,384, provided with an outer wrap jacket and insulating resin, is less functional when sanded or machined to dimension on the Roebel bar because the outer jacket is sanded through and disconnected thereby disrupting the continuity of the outer jacket function.
A non-conducting fiber with a non-conducting epoxy resin that has a conductive fleece over the circumference of the non-conducting felt could be used. The problem, however, comes with applying such a product to the stator bar during pre-consolidation. It molds and shapes to Roebeled stator bar however, it does not allow resin deep into the roebel transposition, leaving void areas. When such a Roebel bar is pressed and cured into shape, the corner edges typically need to be sanded to a radius so that they are not sharp. (electrical stress area). When sanding such a bar to create this radius, the conductive outer fleece is disconnected, interrupting the circuit function of the filler. The outer fleece must make electrical continuity around the circumference of the filler in order to function properly.
Another solution is a paste in a can that uses a conductive epoxy resin (carbon filled) with mica insulating flake in it. This paste is applied with a spatula and when cured on the preconsolidation bar, allows conductive resin to penetrate the Roebel transposition, thereby shorting the insulated copper single turns from one another. Each copper turn is insulated with a polyester/glass yarn for armor protection and separation so that they are isolated in the Roebel stranding. However, this polyester/glass yarn insulation can be impregnated with the conductive resin thereby shorting the strands to one another.
Desired is a solution that overcomes one or more of these prior-art deficiencies. In particular, it would be useful to have a non-conductive resin to penetrate the roebel transposition without having the above listed problems.